NRL chief executive David Gallop maintained the salary cap was a fundamental part of the game amid reports Sonny Bill Williams' legal team was preparing to challenge its validity in court.
In the latest twist in Williams' bid to escape a five-year contract with the Bulldogs so as to sign with French rugby union club Toulon, the $4.1 million salary cap by which all NRL clubs abide has come under scrutiny.

It is believed Williams' lawyers will claim that their client's deal with the Bulldogs is invalid because the salary cap was a restraint of trade.

Williams is still trapped in London today as he awaited a visa to enter France, but it is believed he would start making his way to the French riviera some time tonight.

Gallop said the NRL would not survive without the salary cap.

"It's a foundation stone of the competition and if we were to have a competition without the salary cap, a competition based on pure purchasing power, then a few of our clubs, a small number, would be in a position to buy the best players," Gallop said.

"We would have a lopsided competition and ultimately clubs would be under financial pressure to survive."

"They have a choice not to sign. The restraint of trade argument is a bit of a tawdry one, under the circumstances."

Sports law expert Deborah Healey, a senior lecturer at the University of NSW, said Williams' legal team would need to prove the salary cap was an unreasonable restraint of trade.

This would be dependant on a number of factors, including just how much income was generated by the game.

"Restraints of trade are only okay if they are considered reasonable in all of the circumstances," Healey said.

"They would be arguing that the parts that make up the salary cap are in restraint of trade because they go further than is necessary to protect the interests of the NRL."

Gallop denied players were being denied earning a fair income, adding the NRL and the Bulldogs had received unilateral support from club bosses for seeking an injunction against Williams.

"The salary cap is at a level of affordability and that was the clear message from our clubs today," Gallop said.

"It's not the case that there are large amounts of cash out there that could be paid to players if the salary cap was removed.

"It's set at a level that most of our clubs can afford."

Healey said claims a decision against the NRL salary cap would have an immediate effect of caps in other codes were incorrect.

"A decision in respect to the NRL salary cap would not mean that there was a legal problem with all salary caps," she said.

"Each cap would need to be considered in relation to its own individual circumstances."

Williams' confidant Anthony Mundine, in Newcastle this weekÃƒÂ¢Ã¢Â‚Â¬Ã¢Â„Â¢s super middleweight bout, took time out from his preparations to weigh into the debate.

Mundine and Williams are close, with the former St George star reportedly driving Williams to the airport as he began his great escape on Saturday.

"There should be no salary cap, there's no salary cap to the workers, plumbers, carpenters and builders," he said.

"The game is making millions of dollars from their names. So where is the loyalty from the NRL to the players? There is no loyalty."

Bulldogs chief executive Todd Greenberg said he had yet to receive a reply after writing to both Toulon and the French Rugby Federation in a bid to ward them off inducing Williams to break his deal with the Bulldogs.